This invention relates to the field of colorimetric analysis, more particularly to the analysis of trace levels of metals that form colored, extractable dithizonate complexes, such as heavy metals.
As illustrated by Snyder, Anal. Chem., vol. 19 (9): 684-687 (Sept. 1947), an improved dithizone method for colorimetric analysis of a lead-containing aqueous sample is known. In the method, dithizone is provided as a solution in chloroform, a water-immiscible extractant for the dithizonate complex; ammonium citrate, anhydrous sodium sulfite, potassium cyanide and ammonium hydroxide are employed as reagents; and the final extraction of the multiple extraction procedure is carried out at a pH of 11.5. Snyder uses an amount of dithizone that equals or exceeds more than 3000 .mu.g, and evaluates color with a spectrophotometer.
Exposure to heavy metals continues to be a matter of concern. Exposure to lead in drinking water, regardless whether the lead is in source water or is from corrosion of plumbing materials, needs to be minimized.
Typically, analysis for heavy metals is carried out in certified laboratories. However, such an analysis does not provide an immediate answer, cannot be conducted on site, requires preservation of a sample for later analysis, requires a high level of technical expertise to conduct, and is expensive.
As illustrated by U.S. Pat. No. 4,770,853 to Bernstein, U.K. Patent No. 1,542,411 of Carnie et al, and U.S. Pat. Nos. 3,980,436 to Greenfield et al, 4,786,604 to Michael, and 4,125,376 to Razulis, self-contained, portable devices are known for assays including colorimetric assays.
Bernstein, for example, describes a device for immunodiffusion assay that includes a sample collector, a tube with compartmentalized reagents and frangible seals, and a capture membrane disposed beneath the lower frangible seal to which sample and reagent are transferred from the sample collector by contact.
Carnie et al describe an analyzer for on the spot analysis of substances suspected of being explosives or narcotics. The analyzer includes a sample container connected to one end of a tube, a frangible ampoule containing a reagent, and a piston member which is inserted into the other tube end and employed to crush the ampoule.
The Greenfield et al patent pertains to a device including concentrically disposed tube and sleeve members providing an annular region between the members with a passage thereto that can be opened or closed. The outer tube has a conical bottom.
Michael describes a detector kit for testing for lead concentrations in excess of approximately 5 parts per million. The kit uses a sodium or potassium chromate solution.
The Razulis device is described as useful for colorimetrically detecting water pollutants including heavy metals, in concentrations measured in parts per million. The device is a test tube containing a foam cube impregnated with a detection chemical solution. A solution of dithizone in di-isobutyl phthalate, is utilized.
Also known is a test kit for lead in water, that includes a stock solution of dithizone in carbon tetrachloride, a water-immiscible organic extractant for lead dithizonate, stored with an aqueous preservative solution. The aqueous solution forms an upper layer and the organic solution forms a lower layer of a biphasic liquid. The test kit further includes a plastic squeeze bottle containing an alkaline solution of cyanide as a stock solution, bottles of pH-adjusting solutions, a pipette having a squeeze bulb, a test tube marked with a fill line and having a stopper, a wash bottle for providing wash water, and a comparator. The kit lacks a deactivator for unreacted cyanide.
In use, the pH of a water sample is determined and adjusted; the pipette tip is inserted into the lower layer of the biphasic liquid, the pipette is filled with an amount of the dithizone stock solution appropriate for a single test (equivalent to about 600 .mu.g/60 ml), and the contents of the pipette are transferred to the test tube; and a specified number of drops of the cyanide stock solution for a single test, are squeezed from the plastic bottle into the test tube. Thereafter, the pH-adjusted water sample is added to the test tube until the fill line is reached; the test tube is capped and shaken vigorously, and the contents allowed to separate into upper and lower layers; and the test tube is inserted into the comparator for viewing through tube side walls. The pipette and test tube are washed for reuse in a subsequent test.
Accordingly, there is a need for an improved portable test kit for colorimetric analysis of trace levels of heavy metals in an aqueous sample. The kit should be simple to use. The test kit should provide results quickly and be cost-effective. In those cases where a sample is found to contain a level of metal of concern, the sample could be preserved and submitted to a certified lab.
Such a kit should also be safe to dispose of, and beneficially would provide an improved method for colorimetrically analyzing for trace levels of heavy metals. In particular, there is a need for a portable test kit sensitive for lead levels in the range of 5 to 10 ppb. By the term, "ppb" is meant .mu.g per liter.